Developmental Biology BY1101 Lectures 4 and 5 Cleavage-

Developmental Biology BY1101
P. Murphy
Lectures 4 and 5
The first steps to forming a new organism
Descriptive embryology 2
Cleavage, Gastrulation, Neurulation and Organogenesis
Early animal development can be divided into the following key events
Fertilisation – cleavage – gastrulation – neurulation – organogenesis
Lecture 3 Lecture 4 and Lecture 5
Cleavage-
Very rapid cell division of the fertilised egg
Features:
Cell cycle involves S phase (DNA synthesis) and M phase (mitosis) but virtually skips G
phases (so no growth). This also leads to faster cell division
Embryo does not enlarge- cell division in the absence of overall growth
Cytoplasm is divided up between emerging blastomere cells.
First 5-7 divisions yield a loose grouping of cells called a morula
Fluid filled cavity forms- blastocoel- to give rise to a hollow ball of cells- the blastula
How animals develop is used as part of their classification- it can tell us something about
their evolution. For example the following is a classification based on cleavage pattern.
Radial cleavage :
As seen in vertebrates and echinoderms- also called deuterostomes.
Cleavages are equal and symmetrical. This explains why early development in the
sea urchin (an echinoderm) is similar to vertebrates
Spiral cleavage: As seen in e,g, mollusca, annelids- also called protostomes. The spindle
is at an oblique angle within the cell so cleavage is spiral. Successive cleavage planes are
different. Producing a spiral pattern.
A variety of egg designs underlies differences in early development of animals
Does the egg contain yolk proteins for embryo nourishment?
e.g. amphibians birds mammals X
How much yolk?

e.g. most plentiful in birds, reptiles, many fishes and insects.
less in sea urchins
How is the yolk distributed? Basis of differences between frog and chick.
Cleavage in the frog
The egg has polarity
Upper pigmented side- animal pole; lower yolky side- vegetal pole.
The movement of molecules in the egg -triggered by fertilisation
→ appearance of “gray crescent”
First two cleavages are vertical
Cleavage 3 is horizontal
Yolk content displaces plane of division toward animal pole.
⇒More cells and blastocoel in animal half
Fig 47.7
This stage is the blastula
Cleavage pattern is similar in all animals classified as deuterostomes-
But in eggs with less yolk e.g. sea urchin, the blastomeres are of more even size and the
blastocoel is more centrally located
Cleavage in the chick
•The entire “yolk” of the chicken egg is the egg cell- packed full of yolk proteins.
•The “white” - a protein rich solution bathing the cell- also used for nutrition
•All the cytoplasm is contained in a tiny disk at the animal pole of the yolk.
Cleavage planes cannot penetrate the dense yolk so only the animal pole
cytoplasm is cleaved. This incomplete cleavage is know as meroblastic cleavage
(complete cleavage is called holoblastic)
Cleavage produces a cap of cells sitting on top of yolk- called blastoderm
So the blastoderm in the chick is the equivalent of the blastula in
the frog- the shape is related to egg architecture (yolk content and distribution).
The cavity that forms (blastocoel) divides two layers of cells: the
hypoblast and the epiblast. The embryo will emerge entirely from
the epiblast.
Therefore the embryo will emerge from a simple flat layer of cells

Cleavage in different species can produce a ball or disk of cells. How does that turn into
an organised embryo?
The next step is gastrulation- Literally means “making a gut”
Gastrulation
Marked by extensive cell movement where the cells of the blastula become organised
into three layers. This 3 layered embryo has a primitive gut.
The details of the movements are different in different organisms (again related to the
architecture of the egg) but the outcome is the same- a 3 layered embryo
The 3-layered embryo is called a gastrula
Morula blastula gastrula
A ball of cells hollow ball 3-layered
hollow ball
Gastrulation best studied in the frog.
How could the early embryologists figure out the details of cell movement in a tiny
embryo?
1. Frog embryo is not so tiny
2. The obviously visible poles aid visualization of movements
3. They used methods to mark cells and follow their movements and their fate- cell
lineage marking / cell fate mapping
Cell lineage marking / Cell fate mapping
Mark a cell early in development and observe where it and its descendent cells end up.
This is how cell fate maps are made and how we can
follow the movements and fates of all the cells
Gastrulation occurs in all eumetazoa (animals with an organised body plan)
Animals that do not undergo gastrulation are the most “simple” animals with no organs
e.g. jelly fish.
“triploblastic” animals – gastrulation produces 3 germ layers.
Some animals are diploblastic (e.g. hydra- ;there are only two layers)
The 3 layers formed are called the embryonic germ layers
Ectoderm “ecto”: outer
Mesoderm “meso”: middle
Endoderm “endo”: inside (Note: the colour scheme used here is the classical
scheme for mapping ectoderm (blue), mesoderm (red) and endoderm (yellow))
Eventually these 3 germ layers develop into all the tissues and organs of the adult animal

The sea urchin- gastrulation on a simple scale
The blastula is a single layered hollow ball
1. Gastrulation begins at the vegetal pole- individual cells detach and
enter the blastocoel- first mesenchyme cells (top arrow)
2. The vegetal plate destined to form endoderm- flattens and buckles
inward- invagination (arrow)
3. Invagination deepens to form a tube called the Archenteron-
this is the primitive gut (arrow).
4. Extensions from the mesenchyme cells called filopodia- drag the
archenteron toward the blastocoel wall (arrow)
5. Archenteron fuses with blastocoel wall - forms digestive tube with
mouth and anus- 3 layers - mesenchyme forms future skeleton
Frog gastrulation
Gastrulation is much more complex in the frog because there are more
cells and there are large yolk laden cells in the vegetal pole, but the
process also produces a 3-layered gastrula with an archenteron as a
primitive gut.
In Fig 47.10 panel 1 you see the fate map of where the future
ectoderm, mesoderm and endoderm are located at the start of gastrulation.
Gastrulation begins when a line of cells begin to change shape and
invaginate. This occurs at the earlier site of the gray crescent and is
called the dorsal lip of the blastopore.
Panel 2: Future mesoderm cells and endoderm cells at the site of the dorsal
lip then begin to roll over the lip and involute into the interior of the embryo.

Inside, these cells move away from the blastopore and become organised into layers of
endoderm and mesoderm.
Panel 3: The original cavity- the blastocoel- is collapsed by the movement
and a new cavity is created- this is the archenteron and will form the future
gut. The yolk rich cells are internalised in the region of the future gut.
Meanwhile the remaining cells on the surface that will form the ectoderm
(blue) move down around the surface to completely cover the embryo.
As in the sea urchin the blastopore is the future site of the anus and the
mouth will form at the opposite side.
As a result of all the cell movement the embryo changes shape- elongates as the cells
meet and converge inside the embryo
Convergent extension
-like lanes of traffic meeting and converging - leading to elongation (extension)
Campbell and Reece Fig 47.16
The important features of gastrulation:
Elaborate cell movement.
Differentiation of cells into 3 broad types: ectodermal, mesodermal and endodermal,
placed in the right position with respect to each other.
Allows communication between neighbouring cells that are now set apart as different to
each other.
Mechanisms involved in bringing about gastrulation- what is needed?
• changes in cell motility
• changes in cell adhesion (therefore molecules on the outside of the cells).
Gastrulation may vary in detail from organism to organism but the result
is similar:
the production of a three layered gastrula with organised groups of
cells that interact and co-operate to form the embryo.
The positioning of the cell layers in the gastrula allows cells to interact in new ways.
Gastrulation brought all the cells into the correct position within the embryo to facilitate
local development of the organs- organogenesis.

While gastrulation involves massive cell movements, organogenesis involves more local
interactions between cells and morphogenetic changes in tissue and cell shape- and in
cell arrangement.
Note that cell differentiation is happening continuously throughout these processes and
cells are getting closer to their final characteristics and functions.
Many of the changes in organogenesis are brought about by Induction:- the influence of
one group of cells on another changing the way in which the responding cells develop
(more in lecture 8).
The process of Neurulation is a very special type of organogenesis
• It sets aside the cells for and forms the rudiments of the entire nervous system.
• It is the first event in organogenesis.
The first event in neurulation starts as gastrulation is still underway and the future
mesoderm cells are moving inside the embryo. A particular group of these migrating
mesoderm cells assemble into a rod-like structure along the dorsal midline of the future
embryo.
These special mesoderm cells form the notochord and have very special characteristics-
they have very potent ability to induce neighbouring cells.
Signals sent from the notochord to the ectoderm lying above induce the ectoderm to
thicken and form the neural plate (green on the figure below) the rudiment of the
nervous system
See figure 47.12 and images below- note the notochord and the neural plate
Cross section
Dorsal view of frog embryo
undergoing neurulation
The outer edges of the neural plate then begin to fold upward- forming the neural folds.
Eventually the two sides of the neural folds fuse to enclose the neural tube (see figure
below).
As the neural tube fuses the adjacent surface ectoderm (blue) also fuses to enclose the
neural tube, which forms the rudiment of the brain and the spinal chord. The neural tube
runs the length of the embryo from anterior where it forms the vesicles of the brain.

By the end of gastrulation and neurulation, the embryo has elongated with a head end and
a tail end and has an internalised neural tube running along the dorsal midline. In the
head region this will form the brain chambers and elsewhere the spinal chord.
head tail
Cross-section
As neurulation proceeds, other organs and structures are also starting to form.
A special set of cells form at the dorsal edge of the neural tube- the neural crest cells
(see below).

These migrate away from the neural tube to many destinations and form peripheral
nerves, teeth, skull bones and other cell types.
Note the coelom in the top figure – this is a fluid-filled cavity which forms by splitting
the mesoderm- it is by definition surrounded by mesoderm on all sides
Many of the organs as they form will push into this space- although contained within the
peritoneum.
The somites are another set of structures forming at this time.
The somites are mesoderm cells that form into blocks on either side of the neural tube
(see figure below). These are transitory structures that later dissociate to form different
cell types.
1. The muscle blocks of the body and muscles attached to the skeleton
2. The vertebrae of the back-bone. The cells from within the somite that will form
the vertebrae (blue on lower diagram) leave the somite and migrate around the
notochord and neural tube. The notochord later degenerates; the only remnants
forming the “disks” between the vertebrae.
Blue: cells migrate around
the notochord and neural
tube and form the vertebrae
Red: Muscle cells migrate
around trunk and into the
limb

The somites show that vertebrates are partially segmented.
In organogenesis cells of one germ layer often induce neighbouring cells of another germ
layer to change character and begin to form a tissue or part of an organ-
-this is precisely why bringing the germ layers into the right position during
gastrulation is so vital for development of the embryo.
-More about induction and its molecular basis in lecture 8
Gastrulation in the chick
The process looks very different because the early embryo is a flat disk of cells with
masses of yolk underneath
The entire embryo will form from one cell layer- the epiblast. Gastrulation involves cells
from the epiblast moving underneath through a line at the midline of the disk called the
primitive streak. Some of these involuting cells form endoderm and some form
mesoderm. The primitive streak marks the future anterior-posterior axis of the embryo.
The primitive streak in the chick is equivalent to the blastopore in the frog (site through
which the cells involute).
In the chick, organogenesis proceeds much as it does in the frog but appears different
because the chick embryo is flat and the frog round.
Organogenesis in the chick: To achieve the same “tube within a tube” (gut within the
body wall) structure of other embryos, the edges of the blastodisk (flat chick embryo)
fold ventrally to enclose the gut - but the gut is still joined in the middle of the body to
the yolk in the yolk sac. See Figure 47.13(a) reproduced below
Note the neural tube, notochord, somites and endoderm of the future gut.
Future gut

By 2-3 days of development the rudiments of all the major organs is seen in the
chick embryo. See Figure 47.13(b) below. A movie showing the beating heat of
the chick embryo at this stage was viewed.
A movie of organogenesis in zebrafish was shown
Fertilization
Cleavage
Gastrulation
Neurulation
The figure on the right shows illustrations Organogenesis
of embryos during each of the
developmental processes that we have discussed
Note it is a continuous series of processes
A movie of the continuous processes listed above in zebrafish was viewed again
The story so far- has led us from quite different egg designs to very similar
embryos among vertebrates- as shown in the figure below for frog (Xenopus), chick,
mouse and zebrafish

This was first noted by Karl Ernst von Baer in the early 19th century- he called the stage
when all vertebrate embryos look alike (2 nd row above) the phylotypic stage- typical of
the phylum. At this stage the general characteristics of the phylum are established-
notochord, neural tube, somites and the very beginnings of the limbs.
Von Baer devised some laws to express this observation: he stated in these laws that
general characteristics of a group appear first while those that distinguish a species
appear later
But why is this? He couldn’t really explain it – there is something he didn’t know
but we know today….
….organisms share similar developmental mechanisms.
The mechanisms driving the changes you have just seen will be explored in future
lectures
Lecture 7: cell differentiation
Lecture 8: cell communication
Lectures 9-10: developmental genetics
Key concepts in lectures 4 and 5
1. Cleavage refers to the very rapid cell division of the fertilised egg. It is marked by little
or no protein synthesis and no growth of the embryo.
2. The first divisions form a loose group of cells called a morula.
3. After further divisions a cavity forms (blastocoel) – the embryo is then called a blastula
4. Cleavage is holoblastic if it divides the entire egg into blastomeres (cells) but
meroblastic if only the cytoplasm is divided and the yolk proteins are excluded (e.g.
chick).
5. The appearance of cleavage and the resulting embryo depends on the amount and
distribution of yolk. Cleavage produces a ball or disk of cells. Gastrulation then turns that
ball or disk into an organised embryo.
6. Gastrulation is a fundamental event in all complex animals (eumetazoa) with organs. It
leads to the formation of a 3 germ layered embryo (for triploblastic animals)- the three
germ layers will give rise to all the future organs and structures.
7. The three germ layers, the ectoderm, mesoderm and endoderm contain 3 broadly
different cell types.

8. Gastrulation is characterised by elaborate cell movement so that the three germ layers
end up in the correct position with respect to each other.
9. This sets groups of cells apart as different to each other and allows communication
between neighbouring cells that will be so important in future events- organogenesis.
10. Changes in cell motility and changes in cell adhesion (therefore molecules on the
outside of the cells) are important in bringing about the cell movements of gastrulation.
11. Gastrulation brought all the cells into the correct position within the embryo to
facilitate local development of the organs- organogenesis.
12. While gastrulation involved massive cell movements, organogenesis involves more
local interactions between cells and morphogenetic changes in tissue and cell shape, and
in cell arrangement.
13. Many of the changes in organogenesis are brought about by Induction- cells of one
germ layer often induce neighbouring cells of another germ layer to change character and
begin to form a tissue or part of an organ- underlines the importance of bringing the germ
layers into the right position during gastrulation.
14. The process of Neurulation- a special event in organogenesis that sets aside the cells
for and forms the rudiments of the entire nervous system.
(Make sure you can describe the events involved in neurulation)
15. By the end of gastrulation and neurulation, the embryo has elongated with a head end
and a tail end and has an internalised neural tube running along the dorsal midline. In the
head region this will form the brain chambers and elsewhere the spinal chord.
16. The somites are mesoderm cells that form into blocks on either side of the neural
tube- transitory structures that later dissociate to form different cell types including the
muscle blocks and the vertebrae
17. In the chick, organogenesis proceeds much as it does in the frog but appears different
because the chick embryo is flat and the frog round.
18. The processes that form the embryo (cleavage, gastrulation and organogenesis) occur
as a continuous series of interlinked events- they are not discrete.
19. The early observation by von Baer that all vertebrate embryos look similar at the end
of gastrulation and early organogenesis reflects the fact that molecular and cellular events
that underlie early development are very similar across all animals.
Lecture 4 and Lecture 5: Learning outcomes: you should be able to….
A) State what cleavage is and, using appropriate terminology, describe cleavage in the
frog and differences in the chick.

B) State what gastrulation is and describe its fundamental importance in all complex
animals. Describe gastrulation in frog but be able to discuss the differences between frog
and chick and how the differences in detail are related to structure of the egg- but that the
outcome is the same.
C) Distil the important features of gastrulation from the descriptions and relate this to its
important outcomes.
D) State what organogenesis is and contrast it with the earlier events of gastrulation.
E) Describe neurulation as a very early event in organogenesis and the importance of this
early event as part of establishing the body plan.
Key terms to be familiar with: Cleavage, blastomere cells, morula, blastocoel, blastula,
gray crescent, holoblastic cleavage, meroblastic cleavage, blastoderm, epiblast,
hypoblast, gastrulation, gastrula, fate map, germ layers, ectoderm, mesoderm, endoderm,
invagination, archenteron, blastopore, involution, convergent extension, primitive streak,
Henson’s node,